It is known that good mixing of the reactants plays an important role in gas phase reactions in achieving elevated conversions and selectivities. Examples are the gas phase phosgenation of aromatic or (cyclo)aliphatic polyfunctional amines. In a continuous process the educts are conventionally introduced into a reactor in gaseous form, as described in various patent applications (for example EP-A 676 392, EP-A 570 799, EP-A 289 840, EP-A 749 958). Mixing of the reaction partners to a degree of segregation of 10−3 should proceed within a period of up to 0.5 seconds. The degree of segregation is a measure of the incompleteness of mixing (EP-A 570 799).
Methods for achieving short mixing times are known in principle. Mixing units having moving or static mixing elements are suitable. Static mixing elements are preferred. A range of different possible embodiments is conceivable for the design of static mixing elements, for example using nozzles, flat jet nozzles or venturi nozzles known from combustion technology.
Disadvantages of many designs are elevated pressure loss or an arrangement which results in insufficiently rapid mixing or backmixing in the mixing zone itself or in the reaction chamber. Elevated pressure loss in the mixing element entails increased effort in the preparation of the gaseous educts. A higher pressure is also associated with an increased boiling temperature. This may result in thermal damage to educts on vaporization and in the formation of secondary products.
Insufficiently rapid mixing or backmixing may result in an increased residence time of a proportion of the educts and products, resulting in unwanted parallel or subsequent reactions. Under certain circumstances, another consequence of inadequate mixing is a non-uniform temperature distribution in the reactor. As a consequence, there may be zones in the reactor which are excessively hot, resulting in increased thermal decomposition of the products. Decomposition products form a solid residue, which is deposited on the reactor wall. In this case, it is conventional to provide the reactor with an inliner which, once encrusted, can be exchanged, substantially facilitating reactor cleaning.
It is an object of the present invention to overcome the preceding disadvantages of known processes for the gas phase phosgenation of amines.
This object may be achieved according to the process of the present invention. It has been found that the known disadvantages may be minimized if the mixing element used is a nozzle with precisely specified dimensions which is incorporated coaxially into a tube which opens directly into the reaction chamber as described hereinafter.